1. Field of the Invention
The present invention relates to a method of producing a high tension steel superior in weldability and low-temperature toughness, having yield strength not smaller than 70 kgf/mm.sup.2 and tensile strength not smaller than 80 kgf/mm.sup.2.
2. Description of the Related Art
In recent years, there is an increasing demand for energy. To cope with this demand, a trend or activity is becoming remarkable for building of welded steel structures intended for development and storage of energy resources, such as offshore structures for submarine resource exploitation, sea bottom researching vessels, pressure vessels for storing energy resources, and so forth. In general, these structures are large in size and, hence, constructed from steel members having large thicknesses. Accordingly, the requirement for safety is becoming more significant.
Steel members used for building of such structures are therefore required to have improved weldability and toughness. In case the structure is intended for use in a corrosive condition as in the case of a submarine structure or a crude oil tank, the material must have sufficient resistance to stress corrosion cracking.
A high tension steel having yield strength not smaller than 70 kgf/mm.sup.2 and tensile strength not smaller than 80 kgf/mm.sup.2 (referred to as "HT 80", hereinafter) is produced, for example, by a method using an effect of improving hardenability which effect is obtained by addition of a trace amount of B (boron). More specifically, in this method, excessive addition of hardenability-improving elements such as C, Ni, Cr and Mo is prohibited in order to reduce carbon equivalent which is one of the indices of weldability and, instead, an Al-B treatment or a treatment for reducing N content is conducted to fully utilize the hardenability-improving effect produced by B. The material is then formed into a product steel member through hardening/tempering after reheating or immediately after rolling. These methods are disclosed, for example, in Japanese Patent Examined Publication No. 60-25494 entitled METHOD OF PRODUCING BORON-CONTAINING LOW-ALLOY-TEMPERED HIGH-TENSION STEEL PLATE and Japanese Patent Examined Publication No. 60-20461 entitled THICK HIGH-TENSION STEEL PLATE HAVING HIGH STRENGTH AND TOUGHNESS. In these methods, a tempered martensitic structure or a tempered low bainite structure is obtained as a result of hardening/tempering, thus attaining high strength and toughness.
An Ni-Cu steel (ASTM 710 steel), having improved strength by precipitation hardening effect of Cu, is shown as a high-tension steel having a high tensile strength without relying upon B. This steel is produced by reheating hardening/tempering or reheating normalizing/tempering and is used as the material of high-tension steel member having tensile strength of 60 kgf/mm.sup.2 or so.
The steel production method which relies upon the hardenability improving effect produced by B can reduce the contents of elements such as C, Ni, Cr and Mo so that the weldability is appreciably improved to such a degree as to prevent cracking even when pre-heating temperature is lowered before welding. This method, however, is still unsatisfactory in that it does not allow pre-heating before welding to be completely omitted. In addition, when a steel produced by this type of method is subjected to welding with a small heat input, the hardness of the heat affected zone (HAZ) affected by the welding heat is increased due to the hardenability improving effect produced by B, with the result that the stress corrosion cracking sensitivity is undesirably increased. When this method is applied to production of a member having large thickness, martensitic or lower bainite structure is obtained because of hardenability improvement effect produced by B in a region which is 1/4 the whole thickness from the surface layer Unfortunately, however, toughness is insufficient in the core portion of the sheet due to occurrence of upper bainite structure.